4.4.3.4 - Aqueous electrolysis and its investigation

Ions in aqueous electrolytes

An aqueous electrolyte contains ions from the dissolved compound and ions arising from water.

For example, sodium chloride solution contains sodium ions, $\ce{Na+}$, and chloride ions, $\ce{Cl-}$. Water supplies hydrogen ions, $\ce{H+}$, and hydroxide ions, $\ce{OH-}$. Water molecules break down to produce these ions, which can be discharged during electrolysis.

The ions are free to move in solution. Positive ions move towards the negative cathode, and negative ions towards the positive anode. But arrival near an electrode does not guarantee that a particular ion will discharge: more than one kind of ion is available. The relative reactivity of the elements helps determine which product forms.

This creates an important difference from a melt. Molten sodium chloride has sodium and chloride ions without water, and gives sodium metal at the cathode. Sodium chloride solution can instead give hydrogen there. The solution's name identifies its dissolved compound; it does not mean that every electrode product must come from that compound.

Copper sulfate solution contains $\ce{Cu^2+}$, $\ce{SO4^2-}$, $\ce{H+}$ and $\ce{OH-}$. Some ions remain in solution while others discharge. The prediction rules below concern aqueous solutions of a single ionic compound using inert electrodes. Changing to a reacting metal electrode introduces additional chemistry and is outside that simple product-prediction setting.

Predicting aqueous electrode products

The electrode products depend on the dissolved compound's ions and the ions supplied by water.

At the negative cathode, hydrogen is produced if the metal is more reactive than hydrogen. The metal's positive ions remain in solution. If the metal is less reactive than hydrogen, the metal is produced instead. Thus sodium chloride solution gives hydrogen, whereas copper sulfate solution gives copper.

At the positive anode, oxygen is produced unless the solution contains halide ions, when the corresponding halogen is produced. Chloride gives chlorine, bromide gives bromine, and iodide gives iodine. Sulfate and nitrate solutions give oxygen by discharge of ions from water, rather than making sulfur or nitrogen at the anode.

For copper chloride, copper is less reactive than hydrogen, so copper ions discharge at the cathode. Chloride is a halide, so chlorine forms at the anode. For sodium sulfate, sodium is more reactive than hydrogen, so hydrogen forms at the cathode; sulfate is not a halide, so oxygen forms at the anode.

The two decisions are independent. Knowing the metal reactivity decides the cathode product; it does not decide the anode product. Always state the electrode along with each product so that a correct pair of names is not assigned to the wrong sides.

Hypotheses and an electrolysis investigation

A testable hypothesis connects a predicted product to the chemistry that should cause it.

For required practical 3, one possible hypothesis is: “With inert electrodes, copper sulfate solution will produce copper at the cathode, but sodium sulfate solution will produce hydrogen there, because the metals lie on opposite sides of hydrogen in the reactivity series.” This predicts an observable difference and gives a reason. “Electrolysis will happen” is too vague to distinguish the possible outcomes.

Use a small beaker of the specified electrolyte and two clean graphite electrodes, connected to a low-voltage direct-current supply. Identify the positive and negative leads before switching on. Support the electrodes so they do not touch, and keep their immersed area and separation comparable between trials. Use the same solution volume, stated concentration, voltage and electrolysis time when comparing solution identity. These controls prevent changes in gas visibility or deposition being attributed to several uncontrolled causes.